The 2[4Fe–4S] Ferredoxins

The 2[4Fe4S] ferredoxins, that are members of the ‘clostridial’ and ‘chromatium’ classes of bacterial ferredoxins, have two cuboidal 4Fe4S clusters each in which each iron is coordinated by three inorganic sulfurs and one cysteinyl sulfur. The protein chains (54–80 amino acids) have internal homology so that the environments of each cluster are similar, and the 12.5 A center-to-center separation of the clusters permits electronic interaction. These proteins participate in electron transfer systems, and, characteristically, have very low redox potentials. NH-S hydrogen bonds, peptide dipole orientations, and solvent accessibility are among factors that have an effect on the oxidation–reduction potential. In that regard, a ‘spine’ of water molecules seems to be a conserved structural component linking the β-sheet in each half of the molecule for all the ferredoxins in this class. Site-directed mutagenesis studies have been carried out to analyze redox potentials, stability, intermolecular interactions, and intramolecular electron transfer. 3D Structure Keywords: high resolution structure; internal symmetry; NHS bonds; antiferromagnetic coupling; water spine; low potential iron-sulfur